I need some help.

I was doing the Touring thing at Road America and loosened up my exhaust manifold to cat joint. Lost the nut and washers on one side of the flange and the other side was about halfway off. I got the hardware and made a fix, but the exhaust was still louder than it should have been.

Today I replaced the gasket between the exhaust manifold and cat as well as the hardware and did an oil & filter job. Took the car out for a shakedown and got this hesitation then surge accompanied by the check engine light. When I back off the throttle the light goes out and it takes 30-45 seconds for the cycle to repeat. It does this in every gear always between 2700 and 3000 rpm. What did I do wrong? It seems that there is a slight whistle, but I'm not sure. This happens only under steady throttle, but I can't duplicate it without the car moving.

 

Got the problem solved, thanks to Jerry from the Florida RX7 club.

I pulled the code and found the problem was in the O2 sensor feedback system. I replaced the sensor figuring that I contaminated it with WD-40 when I was changing the manifold to cat gasket. After clearing the codes the problem was solved.